The invention relates to an implant for stabilizing bones or vertebrae. The implant comprises an implant body that is to be fixed by means of at least one screw to the bone or vertebra. The implant body has at least one hole with a threaded portion for receiving the bone screw. A guiding member is provided in the hole, which guides the screw when it is screwed through the threaded portion. In particular, the implant is an intervertebral cage or a bone plate, however, it can be any other implant that is to be fixed to a bone part of the human or animal body by means of a screw extending through a threaded hole.
An intervertebral implant in the form of a cage that is fixed to the adjacent vertebrae by means of screws is described, for example, in US 2009/0030520 A1. The cage can be made of a bio-compatible material such as Titanium or PEEK (polyetheretherketone).
If the cage is made of a bio-compatible plastic material, such as, for example PEEK, a problem may occur in that due to the greater flexibility of such a material compared to a metal, the bore hole for the screw may yield to some extent when the screw is screwed through the threaded portion. This may result in that the torque necessary for screwing the screw through the bore hole is considerably lower as compared to a metallic interconnection. In some clinical applications, for example, in the case of weak and/or osteoporotic bone substance, with such a low torque, there is only a reduced feed-back for the surgeon, which indicates to him how far the screw has already been advanced and whether the screw has the correct orientation.
U.S. Pat. No. 7,618,456 B2 describes an intervertebral implant in the form of a cage that can be made of any physiologically compatible material, preferably however, of an un-reinforced plastic. Un-reinforced plastic is said to be advantageous over fiber-reinforced plastics. Appropriate bone screws consisting of un-reinforced plastic of which the external threads exhibit load bevels of 11° to 14° may be used. Due to the relatively small slope of the load bevel high clamping forces can be achieved, as a result of which radial elongation and the danger of cracking of the plastic are reduced. Also, the bore hole may be in the form of a metal bush fitted with an inner thread for improving anchoring the bone screw in the plastic implant structure. The known implant may also consist partly of plastic and in the bore hole zones of metal.